Messenger page format

Hello,

When looking at messenger home page i notice that in the "Chats" column(on the left side of the page) there are the names of people whom i have recently chatted. To the right of their names i notice one of three things, there is either a blue circle, indicating a new, unread message or there is nothing or there is a circle with their picture in it. What does it mean when a circle with their picture in it appears to the right side of their name? Does that mean they are active on messenger?